The most visible would likely be the sequel to 1993 hit Mrs. Doubtfire, which hadn't begun filming. It's hard to imagine anyone filling Robin's shoes in that role, and Variety indeed reports that the sequel is unlikely to go forward, though screenwriter David Berenbaum reportedly met recently with Williams to go over the script.

Williams also had several projects in the can, with Night at the Museum: Secrets of the Tomb, set to be released December 19, and Merry Friggin' Christmas scheduled to arrive November 7.